The present invention relates to panels with acoustic attenuation and more particularly to those lining the annular fan channels formed in the nacelles of turbo motors, particularly of aircraft.
Such panels form the envelope, on the fan channel side, of the nacelle, behind the air inlet lip and have a structure adapted to attenuate the noise produced by the central portion of the motor surrounded by the nacelle and particularly the noise produced by the fan.
In practice, these panels include a porous core such as a honeycomb structure covered, on the fan channel side, with an acoustic damping layer and, on the opposite side, a rear reflector.
The acoustic damping layer is a porous structure with the role of dissipating, which is to say partially transforming, the acoustic energy of the sound wave passing through it, into heat.
This porous structure can be, for example, a metallic cloth or a cloth of carbon fibers whose weave permits fulfilling its dissipating function.
These acoustic panels must also have sufficient structural properties particularly to receive and transfer aerodynamic and inertial forces and forces connected with supporting the nacelle, toward the nacelle/motor structural connections, and so it is necessary to impart structural properties to the acoustic damping layer.
To this end, there can, as shown in British patent 2 130 963, provide an acoustic damping layer with two components, namely a structural layer, on the honeycomb side and a porous surface layer, or else use as the damping layer a cloth combining both the acoustic function and the structural function by selecting a diameter of the filaments of the cloth giving to this latter a high resistance to forces coupled with good acoustic damping.
For making such panels, there is known the process consisting in producing the annular assembly forming the wall of the fan channel in two interfitting half channels comprising, for each half panel, the following steps:
predeforming a layer of porous structure on a form identical to a half panel, with the help of jaws stretching the porous material to its elastic limit,
acoustically measuring the shape thus produced so as to qualify the mean value of the acoustic porosity,
adapting to the mean value above, the spacing of winding of the carbon filaments adapted to be deposited on the porous layer to constitute a structural layer,
emplacing on a suitable mold the predeformed shape,
then making the half panel by known techniques of winding said carbon fibers, and emplacing the porous core and the rear reflector.
This process has drawbacks.
Thus, the shape to be produced not being a figure of revolution, there exists in the deformed layer inhomogeneous regions, which is to say regions stretched and regions stressed, which degrade the general acoustic quality of the porous structure. The interval of winding the carbon fibers being adapted to the mean value of acoustic porosity of the structure, the inhomogeneous regions introduce variations in the acoustic attenuation of the noise generated by the motor.
Moreover, the presence of interfitting connections of the two half panels introduces two regions of acoustic refraction in the final acoustic panel, which is prejudicial to the quality of damping the noises generated by the motor.
To overcome these drawbacks, there can, as taught by French patent 2 767 411 in the name of the applicant, be emplaced an acoustic damping layer by winding with a porous material present in the form of strip.
This manner of proceeding not only permits avoiding inhomogeneous regions of the porous layer from an acoustical standpoint, as indicated above, in the conventional manner of fabricating the two half panels, but also eliminates the necessity of interfitting, the winding of the porous layer being adapted to form other layers, namely the structural layer, the porous central core, the rear reflector, so as to produce a complete acoustic panel in a single piece, without interfitting.
The absence of interfitting permits increasing the effective acoustic surface of the panel, of decreasing its weight and reducing the time and cost of production.
However, an acoustic panel of such a structure uses constituent strips of the porous layer in direct contact with the flow of aerodynamic fluid in the fan channel. When these strips are made of metallic cloth, they are easily peeled back at their edge in contact with the aerodynamic flow, the more so as they are disposed substantially perpendicular to the direction of said flow.
Thus, not only the acoustic qualities of the panel are degraded, but moreover, the panel itself is degraded and must be changed, which gives rise to maintenance and down time costs of the aircraft.
The present invention has for its object to overcome these drawbacks by improving the winding technique described in FR 2 767 411.
To this end, the invention has for its object a process for the production of a panel with a protected acoustic damping layer, comprising at least one central core with a porous structure covered, on the one hand, with a porous acoustic damping layer and, on the other hand, a totally acoustic reflector, in which at least said porous layer is emplaced by winding or draping, said porous layer being constituted by parallel strips, characterized in that the edges of the strips of the porous layer are positioned facing a strip deposited by winding or draping and containing a thermoplastic, thermohardenable or thermofusible material adapted, by ultimate heating, to ensure the soliderization of the edges of said strips with the adjacent strip.
According to a way of proceeding adapted more particularly to the production of a panel in a single piece, without interlocking, of generally annular shape, the porous layer and said strip containing a thermoplastic, thermohardening or thermosfusible material, are wound or draped on a mold having the shape of the panel to be produced, the porous structure and the total reflector being then emplaced also by winding or draping.
Other panels, convex or concave, or non-annular, can be produced according to the process of the invention and preferably simultaneously from a same mold on which are wound or draped the various layers constituting the panels.
According to the order of laying down the different layers on the mold, there will be obtained a panel with a concave or convex porous layer.
There can thus be produced for example panels constituting pressure reversing doors or panels with interconnections for a nacelle.
According to one embodiment of the process, said strip containing a thermoplastic, thermohardening or thermofusible material is constituted of filaments pre-impregnated with a thermohardening or thermoplastic resin, said strip constituting a structural layer associated with the porous acoustic layer and adapted, which is the case for use of the process of the invention for the production of air inlet acoustic panels for a nacelle, to transfer the dynamic and inertial forces as well as those connected with the support of the nacelle, toward the structural connections.
By filaments, there is meant an assembly of square, round or rectangular cross-section filaments, of strips of filaments, of meshes, of bundles or layers of filaments, of different nature, for example of carbon, glass or xe2x80x9cKevlarxe2x80x9d.
These filaments in the general sense defined above are deposited for example by winding in various ways and are used essentially in the case of a non-joined deposition of the windings of the porous acoustic damping strip, by covering the intervals between windings.
According to a first embodiment, said filaments are wound on the porous layer, so as to be sandwiched between this latter and the porous core, subsequently deposited, for example by winding of a honeycomb structure in the form of a strip.
The windings of the filaments are spaced from each other and only facing and overlapping the intervals between the strips of the porous layer.
According to another embodiment, said filaments are wound first on the mold so as to be located at least facing and overlapping the intervals between the strips of the porous layer which is wound subsequently and which will therefore be located as a sandwich between the structural layer formed by the filaments and the porous structure, identical for example to the preceding case.
Here again, the filaments are separated from each other and are not an obstacle to the accomplishment by the porous subjacent layer, of its dissipating function.
According to still another embodiment which is a compromise between the two preceding ones, said filaments are disposed on opposite sides of the porous layer but always so as to cover at least the intervals between strips of said porous layer.
According to a second embodiment of the process, said strip containing a thermoplastic, thermohardening or thermofusible material is a strip of perforated metal sheet clad with a film of glue and used for example with a porous layer constituted as in the example of the first embodiment of the process.
These strips of perforated metal sheet are deposited by winding according to various manners.
According to a first manner, the strips of perforated metal have a width equal to or less than that of the strips of the porous layer and are disposed in the first instance on the mold with an interval between two successive strips, then the porous layer is deposited in strips facing said intervals between strips of sheet metal.
According to a second embodiment, the strips of perforated metal have a width substantially greater than that of the strips of the porous layer and are disposed in the first instance on the mold with a slight partial overlapping between strips, then the porous layer is deposited such as for example to align each porous strip with a strip of sheet metal, the windings of the porous layer not touching each other.
According to a third embodiment, the strips of perforated metal have a width less than that of the strips of the porous layer, which is first deposited on the mold, such that the windings slightly overlap, then the strips of metal are disposed facing or not the regions of overlap of the strips of the porous layer, these strips not touching each other.
According to a third embodiment of the process, said strip containing a thermoplastic, thermohardening or thermofusible material is constituted in fact by the porous layer itself, which is formed from a cloth of filaments pre-impregnated with a thermohardening or thermoplastic resin. The filaments are thus selected with a diameter giving to the cloth or the like thus constituted, a high resistance to force coupled with a good acoustic dampening, the porous layer forming at the same time a structural layer.
In this third embodiment of the process, the single layer is wound on the mold so as to form strips or windings with a slight mutual overlap.
In all the examples given above, there are obtained panels which acoustic damping layer, which is necessarily turned toward the aerodynamic flow, is protected at the edges of the strips constituting said layer, because these edges are firmly secured on the one hand to a sublayer or an overlayer, according to the method of application, with a connection of the constituents of the layers by means of a suitable resin or glue.
Any peeling of the edges of the strips forming the acoustic damping layer by the aerodynamic flow is thus prevented and the mechanical properties as well as the strength over time of the panels, are substantially reinforced.
The invention also of course has for its object the acoustic damping panels obtained according to the above process, no matter what the context in which they are used.